Robert D. Steele

Dexterity-enhanced telerobotic microsurgery

A telerobotic platform developed in a collaboration between NASA-JPL and MicroDexterity Systems, Inc (MDS) is described in this paper. The lightweight, compact 6 dof master-slave system is precise to better than 10 microns and can cover a workspace greater than 400 cubic centimeters. Current capabilities of the system include manual position control with augmented shared control modes and automatic modes of control of the robot. Simulated force feedback on the master device has been implemented and plans are to integrate force reflection from the slave end effector and evaluate the performance improvements enabled by the telerobot in simulated microsurgical tasks. The telerobot was used in a recent demonstration of a simulated eye microsurgical procedure.

Performance Analysis and Validation of a Stereo Vision System

This paper presents an in-depth performance analysis and validation of a correlation based stereo vision system being used as part of the ongoing 2003 Mars Exploration Rover flight mission. Our analysis includes the effects of correlation window size, pyramidal image down-sampling, vertical misalignment, focus, maximum disparity, stereo baseline, and range ripples. A key element of validation is to determine the stereo localization error both analytically and experimentally. We study both down-range and cross-range error and verify that while camera calibration inaccuracy contributes to both, stereo correlation error affects only the former. Error contributions of subpixel interpolation, vertical misalignment, and foreshortening on stereo correlation are examined carefully. A novel method using bricks with reflective metrology targets and a mast-mounted stereo camera system enabled experimental measurements of the stereo disparity error. The standard deviation of the down-range disparity error was measured at sigma=0.32 pixel for high-resolution 1024times768 camera images. The result is critical in evaluating accurate rover navigation and instrument placement within given error budgets

The modular telerobot task execution system for space telerobotics

A telerobot task execution system that has been developed for space station Freedom applications is described. The modular telerobot task execution system (MOTES) provides the remote site task execution capability in a local-remote telerobotic system. The design addresses the constraints of limited computational power available at the remote site control system while providing a large range of control capabilities. The system provides supervised autonomous control, shared control, and teleoperation for a redundant manipulator. The system is capable of nominal task execution as well as monitoring and reflex motion. A command interpreter similar to one used on robotic spacecraft is used to interpret commands received from the local site. Execution utilizes multiple control modules which execute based upon command parameterization. The system controls a seven degree-of-freedom manipulator.<<ETX>>

Experiments in Contact Control

This article describes the implementation, experimentation, and application of contact control schemes for a 7-DOF Robotics Research arm. The contact forces and torques are measured in the sensor frame by the 6-axis force/torque sensor mounted at the wrist, are compensated for gravity, and then are transformed to the tool frame in which the contact task is defined and executed. The contact control schemes are implemented on the existing robot Cartesian position control system at 400Hz, do not require force rate information, and are extremely simple and computationally fast. Three types of contact control schemes are presented : compliance control, force control, and dual-mode control. In the compliance control scheme, the contact force is fed back through a lag-plus-feedforward compliance controller so that the end-effector behaves like a spring with adjustable stiffness ; thus the contact force can be controlled by the reference position command. In the force control scheme, a force setpoint is used as the command input and a proportional-plus-integral force controller is employed to ensure that the contact force tracks the force setpoint accurately. In the dual-mode control scheme, the end-effector approaches and impacts the reaction surface in compliance mode, and the control scheme is then switched automatically to force mode after the initial contact has been established. Experimental results are presented to demonstrate contact with hard and soft surfaces under the three proposed control schemes. The article is concluded with the application of the proposed schemes to perform a contact-based eddy-current inspection task. In this task, the robot first approaches the inspection surface in compliance control until it feels that it has touched the surface, and then automatically levels the end-effector on the surface. The robot control system then transitions to force control and applies the desired force on the surface while executing a scanning motion. At the completion of the inspection task, the robot first relaxes the applied force and then retracts from the surface.

Sensor-based collision avoidance : Theory and experiments

A new on-line control strategy for sensor-based collision avoidance of manipulators and supporting experimental results are presented in this article. This control strategy is based on nullification of virtual forces applied to the end-effector by a hypothetical spring-plus-damper attached to the objects surface. In the proposed approach, the real-time arm control software continuously monitors the object distance measured by the arm-mounted proximity sensors. When this distance is less than a preset threshold, the collision avoidance control action is initiated to inhibit motion toward the object and thus prevent collision. This is accomplished by employing an outer feedback loop to perturb the end-effector nominal motion trajectory in real-time based on the sensory data. The perturbation is generated by a proportional-plus-integral (PI) collision avoidance controller acting on the difference between the sensed distance and the preset threshold. This approach is computationally very fast, requires minimal modification to the existing manipulator positioning system, and provides the manipulator with an on-line collision avoidance capability to react autonomously and intelligently. A dexterous RRC robotic arm is instrumented with infrared proximity sensors and is operated under the proposed collision avoidance strategy. Experimental results are presented to demonstrate end-effector collision avoidance both with an approaching object and while reaching inside a constricted opening.

Nonlinear contact control for space station dexterous arms

In October 1996, a research and development task was initiated at JPL to develop and demonstrate nonlinear contact control schemes for the dexterous robotic arms planned for the Space Station. This paper reports on the progress made to-date in this task. Specifically, the paper introduces a new class of contact controllers comprised of a nonlinear gain in cascade with a linear fixed-gain PI force controller and PD compliance controller. The nonlinear gains used are simple hyperbolic functions of force error and contact force, respectively. The stability of the closed-loop systems incorporating nonlinear PI and PD controllers are investigated using the Popov stability criterion. Experimental results are presented to demonstrate the efficacy of the nonlinear force and compliance control schemes for a dexterous 7-DOF arm. These results highlight the advantages of the proposed nonlinear contact controllers compared to conventional fixed-gain controllers.

Rover mast calibration, exact camera pointing, and camera handoff for visual target tracking

This paper presents three technical elements that we have developed to improve the accuracy of the visual target tracking for single-sol approach-and-instrument placement in future Mars rover missions. An accurate, straightforward method of rover mast calibration is achieved by using a total station, a camera calibration target, and four prism targets mounted on the rover. The method was applied to Rocky8 rover mast calibration and yielded a 1.1-pixel rms residual error. Camera pointing requires inverse kinematic solutions for mast pan and tilt angles such that the target image appears right at the center of the camera image. Two issues were raised. Mast camera frames are in general not parallel to the masthead base frame. Further, the optical axis of the camera model in general does not pass through the center of the image. Despite these issues, we managed to derive non-iterative closed-form exact solutions, which were verified with Matlab routines. Actual camera pointing experiments over 50 random target image points yielded less than 1.3-pixel rms pointing error. Finally, a purely geometric method for camera handoff using stereo views of the target has been developed. Experimental test runs show less than 2.5 pixels error on high-resolution Navcam for Pancam-to-Navcam handoff, and less than 4 pixels error on lower-resolution Hazcam for Navcam-to-Hazcam handoff

Real-time collision avoidance for 7-DOF arms

The paper presents experimental results that demonstrate a new approach to real-time collision avoidance for 7-DOF arms. The collision avoidance problem is formulated and solved as a force control problem. Virtual forces opposing intrusion of the arm into the obstacle safety zone are computed in real time. These forces are then nullified by employing an outer feedback loop which perturbs the arm Cartesian commands for the inner position control system. The approach is implemented and tested on a 7-DOF RRC arm and a set of experiments are conducted in the laboratory. These experiments demonstrate perturbation of the end-effector position and orientation, as well as the arm configuration, in order to avoid impending collisions The approach is simple, computationally fast, requires minimal modification to the arm control system, and applies to whole-arm collision avoidance.

Experiments in real-time collision avoidance for dexterous 7-DOF arms

The paper presents experimental results that demonstrate a new approach to real-time collision avoidance for 7-DOF arms. The collision avoidance problem is formulated and solved as a force control problem. Virtual forces opposing intrusion of the arm into the obstacle safety zone are computed in real time. These forces are then nullified by employing an outer feedback loop which perturbs the arm Cartesian commands for the inner position control system. The approach is implemented and tested on a 7-DOF RRC arm and a set of experiments are conducted in the laboratory. These experiments demonstrate perturbations of the end-effector position and orientation, as well as the arm configuration, in order to avoid impending collisions. The approach is simple, computationally fast, requires minimal modification to the arm control system, and applies to whole-arm collision avoidance.

Telerobotics for microsurgery

A telerobotic workstation for microsurgery has been developed that enables scaling down motions and filtering tremor in a surgeons hand. The system is compact and light-weight and has the potential for improving the performance of all surgeons and enabling the development of new surgical procedures currently limited by the dexterity of even the most skilful surgeons.